Operation and maintenance of filtration

The most frequently cited maintenance concern for filters is surface and underdrain clogging caused by organic matter, fine silts, hydrocarbons, and algal matter. Common operational problems include:

• standing water;
• clogged filter surface; and
• clogged, inlet, outlet or under-drains

Design Phase Maintenance Considerations

Implicit in the design guidance in the previous sections is the fact that many design elements of filtering systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include limiting drainage area, providing easy site access (REQUIRED), and providing adequate pre-treatment (REQUIRED).

Construction Phase Maintenance

Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). In particular, with filter construction the most important action for preventing operation and maintenance difficulties is to ensure that the contributing drainage area has been fully stabilized prior to bringing the practice on line (this is a REQUIRED practice).

Inspections during construction are needed to ensure the filter practice is built in accordance with the approved design and standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to verify the contractor’s interpretation of the plan is acceptable with the designer. Example construction phase inspection checklists exist for bioretention, media filter systems, and vegetative filter systems.

Post-construction operation and maintenance

Proper maintenance is critical to the successful operation of a filtration practice. Without regular maintenance, filtration system media can become clogged, losing its ability to conduct water at the designed rate. This can lead to stagnant water, mosquito breeding habitat, and reduction or elimination of pollutant removal capacity.

Warning: A maintenance plan clarifying maintenance responsibility is REQUIRED.

Effective long-term operation of filtration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Some important post-construction considerations are provided below.

• A site specific operation and maintenance (O&M) plan that includes the following considerations should be prepared by the designer prior to putting the stormwater filtration practice into operation:
  • operating instructions for drawdown valves, gates and removable weirs (surface filters only);
  • vegetation maintenance schedule;
  • inspection checklists; and
  • routine maintenance checklists
• A legally binding and enforceable maintenance agreement should be executed between the facility owner and the local review authority to ensure the following:
  • sediment should be cleaned out of the sedimentation chamber when it accumulates to a depth equal to ½ the total depth to the outlet, or when greater than 1.5 feet, whichever is less. The sediment chamber outlet devices should be cleaned/repaired when drawdown times exceed 36 hours. Trash and debris should be removed as necessary; and
  • silt/sediment should be removed from the filter bed when the accumulation exceeds one inch. When the filtering capacity of the filter diminishes substantially (i.e., when water ponds on the surface of the filter bed for more than 48 hours), the top few inches of discolored material should be removed and replaced with fresh material. The removed sediments should be disposed in an acceptable manner (i.e., landfill).
• Media filters that have a grass cover should be mowed as needed during the growing season to maintain maximum grass heights less than 12 inches.

Example operation and maintenance checklists exist for bioretention, media filter systems, and vegetative filter systems. The following table provides a more detailed checklist of maintenance activities and associated schedule along with RECOMMENDED maintenance standards.

Recommended maintenance activities for media filters (Sources: WMI, 1997; Pitt, 1997).
Link to this table

Activity	Schedule
If filter bed is clogged or partially clogged, manual manipulation of the surface layer of sand may be required. Remove the top few inches of media, roto-till or otherwise cultivate the surface, and replace media with like material meeting the design specifications. Replace any filter fabric that has become clogged.	As needed
Ensure that contributing area, facility, inlets and outlets are clear of debris. Ensure that the contributing area is stabilized and mowed, with clippings removed. Remove trash and debris. Check to ensure that the filter surface is not clogging (also check after storms greater than about 1”). Ensure that activities in the drainage area minimize oil/grease and sediment entry to the system. If permanent water level is present in pre-treatment chamber (e.g., perimeter sand filter), ensure that the chamber does not leak, and normal pool level is retained.	Monthly
Check to see that the filter bed is clean of sediment and the sediment chamber is not more than 6 inches of sediment. Remove sediment as necessary. Make sure that there is no evidence of deterioration, spalling or cracking of concrete. Inspect grates (perimeter sand filter). Inspect inlets, outlets and overflow spillway to ensure good condition and no evidence of erosion. Repair or replace any damaged structural parts. Stabilize any eroded areas. Ensure that flow is not bypassing the facility. Ensure that no noticeable odors are detected outside the facility.	Annually
Remove and replace the top 2-5 inches of media every 3 to 5 years for low sediment applications, more often for areas of high sediment yield or high oil and grease	3 to 5 years

Caution: The filter media should consist of an 18-inch layer of clean washed medium sand (meeting ASTM C-33 concrete sand) on top of the underdrain system.